linux-kernel - Re: [PATCH v2 1/1] mm/vmalloc: Move draining areas out of caller context

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Date:   Tue, 25 Jan 2022 20:17:40 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Uladzislau Rezki <urezki@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH v2 1/1] mm/vmalloc: Move draining areas out of caller
 context

On Tue, Jan 25, 2022 at 06:46:35PM +0000, Matthew Wilcox wrote:
> On Tue, Jan 25, 2022 at 06:12:48PM +0100, Uladzislau Rezki wrote:
> > On Tue, Jan 25, 2022 at 04:50:14PM +0000, Matthew Wilcox wrote:
> > > On Tue, Jan 25, 2022 at 05:39:12PM +0100, Uladzislau Rezki (Sony) wrote:
> > > > @@ -1768,7 +1776,8 @@ static void free_vmap_area_noflush(struct vmap_area *va)
> > > >  
> > > >  	/* After this point, we may free va at any time */
> > > >  	if (unlikely(nr_lazy > lazy_max_pages()))
> > > > -		try_purge_vmap_area_lazy();
> > > > +		if (!atomic_xchg(&drain_vmap_work_in_progress, 1))
> > > > +			schedule_work(&drain_vmap_work);
> > > >  }
> > > 
> > > Is it necessary to have drain_vmap_work_in_progress?  The documentation
> > > says:
> > > 
> > >  * This puts a job in the kernel-global workqueue if it was not already
> > >  * queued and leaves it in the same position on the kernel-global
> > >  * workqueue otherwise.
> > > 
> > > and the implementation seems to use test_and_set_bit() to ensure this
> > > is true.
> > >
> > It checks pending state, if the work is in run-queue you can place it
> > one more time. The motivation of having it is to prevent the drain work
> > of being placed several times at once what i see on my stress testing.
> > 
> > CPU_1: invokes vfree() -> queues the drain work -> TASK_RUNNING
> > CPU_2: invokes vfree() -> queues the drain work one more time since it was not pending
> 
> But why not unconditionally call schedule_work() here?
>
We can :) The question is do we agree that extra queuing will be kind of
spurious? Because the CPU_1 will complete all cleanups once it is physically
on CPU and others workers just bail out.

We can disregard those spurious wake-ups for sure. If someone complains about
it in the future we can think later then.

Re-spin and do it unconditionally? I do not have a strong opinion about it.

--
Vlad Rezki